The present invention relates to a variable bitrate video coding method and to a corresponding video coder allowing to carry out said method.
As described in the document  less than  less than MPEG Video coding: a basic tutorial introduction greater than  greater than , S. R. Ely, BBC Research and Development Report, BBC-RD-1996/3, pp. 1-10, MPEG activities started in 1988 with the aim of defining standards for digital compression of video and audio signals. The first goal was to define a video algorithm for digital storage media such as the CD-ROM (Compact Disc Read-Only Memory), but the resulting standard was also applied in the Interactive CD system (CD-I). Allowing transmission and storage of picture data at bit rates in the range of 1 to 15 Mbits/s, this standard is based on a data compression achieved by using a block-based motion compensation for the reduction of the temporal redundancy and a discrete cosine transformation (DCT) for the reduction of the spatial redundancy.
With conventional CD standards such as CD-I and CD-ROM, the transmission bit rate is fixed and pictures can therefore only be coded at a constant bit rate. New standards such as the Digital Versatile Disc (DVD) allow for transmission of data at a variable bit rate (VBR): complex scenes can be coded at a higher bitrate than scenes containing less information, in order to maintain a constant quality.
An object of the invention is to propose a VBR video coding method allowing to obtain such a constant quality of the coded sequence, but under the constraint of a given total bit budget (i.e. according to a predetermined target bitrate).
To this end the invention relates to a VBR video coding method characterized in that it comprises:
(a) a coding step, for coding a picture sequence with a constant quantization step and generating a bitstream for which the average bit rate of the sequence does not fulfil necessarily a required constraint of a specific size;
(b) a prediction step, for predicting the quantization stepsize which must be used to code the bitstream according to a predetermined target bit rate;
(c) at least one repetition of said coding and prediction steps in order to get a more precise estimation of said stepsize;
(d) a final control step, relying on a comparison of the cumulative bit rate for all previous frames with the cumulative predicted bit rate, in view of an adjustment of the stepsize so that said target bit rate is matched.
Another object of the invention is to propose a VBR video coder provided for implementing said coding method.
To this end the invention relates to a variable bit rate video coder characterized in that it comprises:
(a) a coding branch, including in series at least a quantization circuit and a variable length circuit;
(b) a prediction branch, including in series at the output of said quantization device at least an inverse quantization device and a prediction sub-system and defining by subtraction the signal to be coded by said coding branch;
(c) a control circuit provided for carrying out the implementation of the following operations:
a coding operation of a video sequence with a constant quantization stepsize;
a prediction operation for an estimation of a quantization stepsize allowing to code the bitstream with a specified target bitrate;
at least one repetition of said operations;
a final controlling operation in order to match the total number of bits spent during the coding operation to said target bitrate.